September takes the garden from summer to fall, with plants growing more slowly or dying back, late season flowering, color changes, fruits and seeds maturing, early signs of animal migration, and shorter days. Native plant gardening this month includes: collecting seed, planting, weeding, trimming, monitoring plants and pollinators, and reflecting on the summer season.
This summer will be remembered for garden maintenance in the face of rain, heat, and mosquitoes. Since the beginning of May, the Madison area has received rainfall exceeding the long term average. Despite a less rainy period in July through mid-August, by late August we had already received more precipitation (36 inches) than the total average amount for the entire year (34 inches). The pattern for temperature shows multi-day heat waves in May, June, and late August. Even brief periods of extreme heat can alter phenology and shorten bloom times. This affects pollinators and may reduce the number of seeds produced.
These heat and moisture patterns led to dramatic plant growth in the gardens. Species in medium or moist soils grew taller. Rhizomatous species, spreading by underground stems, covered more area. For example, blue-joint grass (Calamagrostis canadensis) and prairie cord grass (Spartina pectinata) expanded higher up slopes in our rain gardens. Weeds grew taller and faster; giant ragweed (Ambrosia trifida) was 12 feet tall in some spots, and foxtails (Setaria spp.) germinated and grew rapidly last month on bare disturbed spots.
The large rain garden north of the Visitor Center handled the summer’s heaviest storm remarkably well. After 6 inches of rain fell at the Arboretum on Aug 20, the rain garden basin filled completely. Twenty-four hours later, the water level had dropped 1 foot, through transpiration from plants and infiltration into the soil. During the next twenty-four hours, the level dropped another foot, and by the third day, no standing water remained. Plants were not damaged during the short period of high water and soon there was ample capacity to handle later storms. Do you have a rain garden that helps prevent runoff and flooding nearby?
We’ve come to consider monitoring an essential gardening practice, as plantings mature and change. During August, we documented all plant species growing in each of our garden areas. For example, more than 75 plant species grow in the large rain garden described above, and more than 105 species in the Homeowner’s Demonstration and Children’s Garden area. Also, we continue to track bumble bees and other pollinators in frequent photo surveys. Nine bumble bee species (including the endangered rusty-patched bumble bee) have been confirmed in the garden so far this season. How is plant or animal species diversity changing in your garden?
No matter the conditions or composition, gardens thrive with care and stewardship. Many thanks to our student native plant gardeners, Ali and Saige, for the myriad ways they contributed to garden success this summer. Also, some native plant garden volunteers returned to school this month as teachers or students—we enjoyed working with all of you. Fortunately, we’ll have six or more weeks of gardening with our continuing volunteers, weather permitting. Early fall is a wonderful time to enjoy our garden and yours.
—Susan Carpenter, Arboretum native plant gardener